Actually, modern ceramic capacitors generally are pretty high Q, though
I wonder if the "Hi-Q" may be a trademarked name. The problem, as Roy
suggests, is more likely to be temperature stability, and as he says,
the 5pF and 15pF parts in the design are the ones to be careful about.
I'd guess you will have a very hard time finding 5pF and 15pF
capacitors which are NOT either temperature stable (NPO or C0G type) or
controlled coefficient (negative or positive temperature coefficient,
typically used to compensate for drift in coils...since a coil
typically expands with increased temperature, its inductance goes up,
and that can be compensated by a capacitor whose capacitance goes
down). The dielectrics which are not stable, like X7R, Y5V and Z5U,
are used in high capacitance parts, and looking at a couple
manufacturers' data sheets, the smallest value X7R I see is 100pF. If
the capacitor is large enough, it may be marked "NPO" or "N220" or
something similar to let you know its temperature characteristic.

In the RF part of the circuit, especially, keeping lead lengths very
short is important. You don't want the part that's supposed to be a
capacitor also include a lot of inductance.

There are ceramic capacitors which are specially designed for use at
UHF and microwave frequencies, and maintain very high Q up into GHz
frequencies, but you shouldn't need anything like that. Also, be aware
that "5pF" is deprecated: modern parts are much more likely to be in
the standard "e12" sequence: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9,
4.7, 5.6, 6.8, 8.2, 10, ... So for the "5pF" you'd use a 4.7pF part
(and probably not notice the difference at all).

Another note: I notice the instructions you referenced say to use a
form for the source coil that's EXACTLY 8mm diameter. I'm confident
that if you use one that's 5/16" diameter, you won't notice the
difference. In fact, if you use one that's 1/4" diameter and wind it
with 32 turns of close-wound 24 AWG enamelled wire, it's very likely to
work just as well. You could also use a 2.2uH commercial RF inductor.

So maybe you were only baring your soul...

Cheers,
Tom


SparkySKO wrote:
Hello All!

Bare with me, I'm a newbie.

I'm looking into building the Radio Shack Special

http://www.electronics-lab.com/projects/rf/007/

And looking through the parts list, I see that he references using Hi-Q
Ceramic Capacitors. This radio is for FM, so 90-110mhz.

I have a bunch of normal ceramic capacitors, but no hi-q capacitors. Is
there really much of a difference between the two, or am I likely to
fail if I don't use them? I'd hate to solder all this onto a board, and
risk messing up the board when removing them.

Any advice on the difference between them would be helpful. I'm
guessing they oscillate better, or at higher frequencies.